Page 247 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Although about half of all pumps manufactured in the world are
centrifugal (the other half are positive displacement), industry tends to
use a higher quantity of centrifugal pumps. For that reason, much of
this book has dealt with pump theory, applications, and problems, from
a centrifugal point of view. You may think that we have abandoned PD
pumps in this book. You would be wrong.
Actually, everything we said about bearings, mechanical seals, piping,
TDH, system curves and mating the pump curve to the system curve,
the affinity laws, cavitation, horsepower and efficiency are as applicable
to PD pumps as centrifugal pumps.
So in this chapter of failure analysis and corrective methods, we decided
to consider some problems, symptoms, and remedies particular to I’D
pumps. We’re using two tables. The first table lists the few symptoms
that send a I’D pump into the shop. These symptoms are mated to
another column of possible causes listed in numerical order. The
numerical causes are on the second table starting with the source of the
problem in the left column and the probable cause/suggested remedy
in the right column. As you go through the list, you’ll see again that
PD pumps and centrifugal pumps have a lot in common. Enjoy.

SYMPTOMS AND CAUSES OF FAILURE FOR POSITIVE DISPLACEMENT PUMPS
Symptom Possible Cause
Pump fails to discharge liquid. 1,2,3,4,5,6,8,9

Noisy pump. 6,10,11 ,I 6,17,18,19
Pump wears rapidly. 11,12,13,16,20,23
Pump not up to capacity. 3,5,6,7,9,21,22
Pump starts, then loses suction. 1,2,6,7,10
Pump consumes excessive power. 14,16,17,20

Source of Problem Suggested CauselRemedy
1. Suction problem. Not properly primed
2. Suction problem Suction pipe not submerged

3. Suction problem. Clogged strainer
4. Suction problem. Foot valve leaks
5. Suction problem. Suction lift too high
6. Suction problem. Air leak in suction piping
7. Suction problem. Suction piping too small